Mechanistic analysis of shale permeability evolution data

Constant confining pressure (CCP) tests and constant effective stress (CES) tests were widely conducted to measure shale permeability. The permeability data were explained by two types of permeability models, including apparent permeability models based on the slippage effects and poroelastic permeability models based on effective stress. In these experiments and models, the basic assumption is that the slippage effects, effective stresses and gas sorption-induced matrix swelling/shrinking are the reasons that cause shale permeability change, and they could be separated and investigated individually. In order to see if this basic assumption was appropriate, we collected shale experimental permeability data measured under the CCP and CES conditions; as well as their comparison with solutions of the poroelastic theory and the apparent permeability theory, respectively. A conceptual model of shale permeability evolutions was built. It's found that for both CCP and CES tests permeability ratios are primarily determined by the transient effective stresses in shale with well-developed macro-fractures, or the slippage effects and the transient effective stresses in shale with less-developed macro-fractures. For shale samples with the effective flow radius of pores is smaller than 5µm (initial pore pressure=1.0MPa), the apparent permeability theory can be used to explain the permeability. The permeability ratio is bounded by an upper envelope which is corresponding to the combined solution of free-swelling and slippage effects (with the increase of pore pressure the k/k₀first<1 then rebound to >1 for CCP test, while for CES test the k/k₀<1) and a lower one which is composed by zero-swelling and slippage effects (with the increase of pore pressure the k/k₀<1 for both CCP & CES test). For shale samples with well-developed macro-fractures, the apparent permeability theory could not be used to explain the permeability data. Just like coal, the permeability ratio is also bounded by an upper envelope which is corresponding to the solution of free-swelling (with the increase of pore pressure the k/k₀>1 for CCP test, while for CES test the k/k₀=1) and a lower one which corresponds to zero-swelling (with the increase of pore pressure the k/k₀<1 for both CCP & CES test). Through these comparisons, we found that permeability data for both types of tests are confined within the solutions for two extreme boundary conditions: free-swelling for the upper bound, and zero-swelling and slippage effects combined for the lower bound. These findings suggest that permeability ratios for both CCP tests and CES tests are primarily determined by the matrix-fracture (or pores and dense matrix block) interactions, including sorption-induced swelling/shrinking, through transient effective stresses in matrixes and fractures (or pores and dense matrix block). This non-equilibrium seepage process is very important for shale gas extraction.